Studies have been conducted in the two previous years to evaluate the acute and short-term toxicity of arsine gas. Fischer 344 rats, B6C3F1 and C57BL/6 mice, and Syrian golden hamsters have been exposed to arsine gas at concentrations of from 10 ppb to 50 ppm for periods ranging from 5 hours to 90 days. All groups exposed to a single 6 hour exposure of 25 ppm experienced 100% mortality while those exposed to 5 ppm for four weeks or 2.5 ppm for 13 weeks showed no overt signs of toxicity. Urine samples from these studies showed increased levels of coproporphyrin and 7 and 8 carboxyl uroporphyrin. This data suggests that alterations in the heme biosynthetic pathway as reflected in increases of specific species of urinary porphyrins may be used as early biological indicators of ongoing arsine toxicity. A method has been developed to improve the measurement of specific porphyrin species in rodent urine to further refine this model. Tissues from these exposures are being analyzed for arsenic content to provide a measure of the tissue dose for specific exposure regimens.